The invention relates generally to boron nitride fibers and more particularly to a process for producing integral boron nitride fiber mats.
Boron nitride (BN) possesses a number of highly desirable properties which render it useful in a wide variety of applications. Its high electrical resistivity, coupled with its high thermal conductivity, make it especially useful in electrical and electronic applications requiring a material which simultaneously acts as an electrical insulator and a thermal conductor. Its excellent thermal shock resistance renders it effective as a refractory at temperatures up to 1600.degree. C. or higher in a non-oxidizing atmosphere and at temperatures as high as 700.degree. to 900.degree. C. in air. It is highly corrosion resistant, being inert to most organic liquids and many corrosive chemicals and displaying excellent resistance to attack by various molten metals. Furthermore, because of its low dissipation factor over a wide temperature range, this material is well suited for use in microwave and radar dielectric components (radar windows).
Various methods for the manufacture of boron nitride fibers are known in the prior art; for example, it is disclosed in U.S. Pat. No. 3,429,722, issued to James Economy et al., that boron nitride fibers can be manufactured by heating boron oxide fibers in an ammonia atmosphere, a process known generally as nitriding.
While it is well known in the prior art that boron nitride fibers can be manufactured having good characteristics, the use of such fibers has been limited due to difficulties in forming three dimensional articles from the fibers. U.S. Pat. No. 3,816,242 to Selover et al. teaches a process for binding BN fibers into a felt using an aqueous solution of an inorganic, water soluble material as the binder. However, almost any substance other than BN which is used to bond the fibers to each other has properties which are inferior to the properties of the boron nitride fibers, thus resulting in a bonded article which is unsuitable for use in many applications. For example, when a boron nitride fiber mat, which is bound by prior art materials, is used as an electric cell separator in a lithium sulfide battery utilizing a corrosive cell electrolyte, such as molten lithium chloride or potassium chloride, the fibers separate from each other due to the inability of the binding material to withstand the high temperature corrosive environment.
An attempt has been made prior to the present invention to form articles from boron nitride-bonded boron nitride fibers, for example, by heating boron nitride fibers impregnated with boric acid solution to elevated temperatures in ammonia as disclosed in U.S. Pat. No. 3,837,997 to James Economy et al.
In addition to the above-noted references relating to boron nitride fibers, shaped boron nitride, usually non-porous, bodies have also been prepared in the past. Such articles are disclosed, for example, by Taylor, U.S. Pat. No. 2,888,325, which teaches the use of a multiple stage nitriding process comprising intermittent addition of oxygen containing boron compound at intermediate stages of nitriding, followed by further nitriding.
Furthermore, such articles have been prepared by sintering boron nitride fibers in the presence of boron oxide.
U.S. Pat. No. 4,130,631, assigned to the same assignee as the present application, does disclose a method for producing a non-woven porous boron nitride fiber mat having sufficient strength for use as an electric cell separator in the above mentioned molten lithium chloride environment. However, despite the superior physical characteristics of the resulting product, this method is not sufficiently detailed to insure an economically feasible operation.
Therefore, it is an object of the present invention to provide a totally integrated manufacturing process to produce boron nitride fiber mats starting from the basic boron oxide, and to do so in a commercially feasible manner.